Position detection of mechanical resonant scanner mirror

ABSTRACT

Two piezoelectric sensors are mounted on the back of a spring-plate of a mechanical resonance scanner on respective sides of a center line coinciding with an axis of rotation. As the scanner mirror rotates back and forth the two sensors are accelerated and decelerated at a 180° phase difference. Each sensor&#39;s output voltage crosses a zero level when the acceleration is unchanging. A differential amplifier detects the zero crossings for motion along the axis of rotation. Common mode rejection eliminates the non-rotational accelerations associated with external vibrations and shocks, and prevents masking the mirror&#39;s zero-crossings.

BACKGROUND OF THE INVENTION

This invention relates to optical scanner devices, and more particularlyto a mechanical resonant scanner having a mirror which moves to deflectlight along a scanning pattern.

Mechanical resonant scanners are used in retinal display devices to scanan image onto the retina of an eye. In an exemplary configuration onescanner is used to provide horizontal deflection of a light beam, whileanother scanner is used to provide vertical deflection of the lightbeam. Together the two scanners deflect the light beam along a rasterpattern. By modulating the light beam and implementing multiple colors,a color image is scanned in raster format onto the retina.

Scanning rate and physical deflection distance characterize the movementof the scanner's mirror. In the context of a retinal display thescanning rate and deflection distances are defined to meet the limits ofthe human eye. For the eye to continually perceive an ongoing image thelight beam rescans the image, or a changing image, in periodic fashion.Analogous to refreshing a pixel on a display screen, the eye's retinalreceptors must receive light from the scanning light beam periodically.The minimum refresh rate is a function of the light adaptive ability ofthe eye, the image luminance, and the length of time the retinalreceptors perceive luminance after light impinges. To achieve televisionquality imaging the refresh rate is to be at least 50 to 60 times persecond (i.e., ≧50 Hz to 60 Hz). Further, to perceive continuous movementwithin an image the refresh rate is to be at least 30 Hz.

With regard to the deflection distance, the mirror is deflected todefine a raster pattern within the eye. System magnification anddistance between the scanner and an eyepiece determine the desireddeflection distance.

To define a raster pattern in which millions of bits of information(e.g., light pixels) are communicated onto a small area (i.e., eyeretina), the position of the mirror needs to be known to a high degreeof accuracy. In a mechanical resonant scanner, the resonant frequencydefines the scanning rate. The resonant frequency is determined by anatural frequency of the scanning structure. Conventionally, amechanical turn-screw is used to tune the resonant frequency to be equalto an image data drive signal (e.g., HSYNC or VSYNC). The resonantfrequency, however, changes with environmental changes (e.g.,temperature, barometric pressure). This change in resonance changes thephase relationship between the phase of the image data drive signal andthe position phase of the mirror position. Accordingly, there is a needto monitor the position of the mirror.

SUMMARY OF THE INVENTION

According to the invention, two piezoelectric sensors are mounted on aspring-plate of a mechanical resonance scanner. The spring-platesupports a mirror or has a polished surface embodying a mirror used fordeflecting a beam of light.

According to one aspect of the invention, the two piezoelectric sensorsare mounted on respective sides of a center line on the back of thespring plate. Such center line is in parallel with the mirror's axis ofrotation. As the mirror rotates back and forth the two sensors areaccelerated and decelerated generating sensor output voltages at a 180°phase difference. A sensor output voltage crosses a zero level when theacceleration is unchanging. Both sensors cross the zero level at thesame time but with opposite voltage polarity swings.

According to another aspect of the invention, a differential amplifieror other device detects the zero crossing. Such zero crossingscorrespond to the mirror being at a known position. Specifically, themirror undergoes zero acceleration at its maximum velocity. Maximumvelocity occurs when the mirror is at a level orientation relative toits mirror support structure. Detection of the zero crossovercorresponds to the mirror being at this known position.

According to another aspect of this invention, acceleration of thescanner as a whole is differentiated from the accelerations of themirror within the scanner. The piezoelectric sensors respond toacceleration to define a voltage output signal. In one application thescanner is part of a virtual retinal display worn by a user. Such useris able to move with the scanner. Such motion or other externalvibrations or shocks induce voltage onto the piezoelectric sensors. Byprocessing the two piezoelectric sensor output signals at a differentialamplifier the common modes of the respective sensors are canceled out.Such common mode rejection eliminates the non-rotational accelerationsassociated with the external vibrations and shocks, and prevents maskingthe mirror's zero-crossings.

According to another aspect of the invention, the phase of an image datadrive signal used for feeding image data onto the light beam beingreflected by the scanner is locked to the position phase of the mirroroscillation action.

According to one advantage of the invention, detection of when themirror is at the known position is useful for identifying phasedifference between the phase of the image data drive signal and theposition phase of the mirror. Mirror position phase changes are caused,for example, by changes in temperature. The resulting phase differenceis corrected to keep the drive signal and mirror oscillation in phase.By doing so, a uniform raster scanning pattern is defined by one or morescanners. These and other aspects and advantages of the invention willbe better understood by reference to the following detailed descriptiontaken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a mechanical resonant scanner accordingto an embodiment of this invention;

FIG. 2 is an exploded view of the scanner of FIG. 1;

FIG. 3 is a partial perspective view of the scanner of FIG. 1 showingmagnetic fields for oscillating the scanner mirror;

FIG. 4 is a diagram of the extreme deflection positions of the scannermirror;

FIG. 5 is a perspective view of the scanner spring plate and mirrorposition sensors;

FIG. 6 is an optical diagram of a virtual retinal scanner including amechanical resonant scanner of FIG. 1; and

FIG. 7 is a circuit block diagram of a circuit for locking scanner drivesignal phase to the phase of a mirror's oscillation.

FIG. 8 is a circuit block diagram of a phase locking circuit in analternative embodiment in which a pixel clock signal is derived.

DESCRIPTION OF SPECIFIC EMBODIMENTS

Scanner Overview

FIGS. 1 and 2 show a mechanical resonant scanner 10 having a mirror 12.The mirror 12 is formed integral to or separate from a spring plate 14.In one embodiment the mirror 12 is formed by a smooth, polishedreflective surface area of the spring plate 14. In another embodimentthe mirror 12 is a separate structure mounted to the spring plate 14.The scanner 10 also includes permanent magnets 18, 19 which createmagnetic circuits for moving the mirror 12 at a high oscillatingfrequency about an axis of rotation 16. In one embodiment the onlymoving part is the spring plate 16 with mirror 12.

The resonant scanner 10 also includes a base plate 20. A pair of statorposts 22 and magnets seats 24 are formed on the base plate 20. The baseplate 20, stator posts 22 and magnet seats 24 preferably are formed ofsoft iron. In one embodiment the base plate 20 is elongated with amagnet seat 24 formed at each end. Each magnet seat 24 includes a backstop 32 extending up from one end of the seat 24 and a front stop 34extending up from an opposite end of the seat. The stator posts 22 arecentrally located between the magnet seats 24. Respective stator coils26 are wound in opposite directions about the respective stator posts22. The coil 26 windings are connected either in series or in parallelto a drive circuit which tunes the oscillating frequency of the mirror12.

The spring plate 14 is formed of spring steel and is a torsional type ofspring having a spring constant determined by its length, width andthickness. The spring plate 14 has enlarged opposite ends 42 and 44 thatrest directly on a pole of the respective magnets 18, 19. The magnets18, 19 are oriented such that they have like poles adjacent the springplate. For example, the North poles of the magnets 18, 19 are adjacentto the spring plate 14 in one embodiment while the South poles of themagnets 18, 19 are adjacent to the base plate 20. Narrower arm portions46, 48 of the spring plate 14 extend from each of the enlarged ends 42,44 to an enlarged central mirror portion 12 of the spring plate 14. Themirror 12 forms an armature for the resonant scanner 10 directly overthe stator posts 22. The mirror 12 axis of rotation 16 is equidistantfrom each of the two the stator posts 22.

The spring plate 14, magnets 18, 19 and base plate 20 are tightlyclamped together by respective spring plate caps 52, 58. Each cap 52, 58is formed as a block with an opening 60/62. The respective opening 60/62is formed so that each respective cap 52/58 can accommodate a springplate end 42/44, a magnet 18/19 and a magnet seat 24, as well as part ofa spring plate arm 46/48. Cap 52 is held securely to the base plate 20by a pair of screws 54, 56 so as to clamp the spring plate 14 and magnet18. The screws 54, 56 extend up through apertures 58 in the base plate20 on opposite sides of the magnet seat 24 and into threaded aperturesformed in the cap 52 on opposite sides of the opening 60. The cap 58 issimilarly clamped to the base plate 20 by respective screws 61, 63 thatextend up through respective apertures 64 and into threaded aperturesformed in the cap 58 on opposite sides of the cap opening 62.

Magnetic Circuits

FIG. 3 shows magnetic circuits formed in the mechanical resonant scanner10. The magnetic circuits cause the mirror 12 to oscillate about theaxis of rotation 16 (see FIG. 1) in response to an alternating drivesignal. An AC magnetic field 63 is caused by the stators 22. DC magneticfields 65, 67 are formed (i) between magnet 18 and one stator 22, (ii)between magnet 19 and such one stator 22, (iii) between magnet 18 andthe other stator 22, and (iv) between magnet 19 and the other stator 22.A first magnetic circuit extends from the top pole of the magnets 18 tothe spring plate end 42, through the arm 46 and mirror 12, across a gapto one of the stators 22 and through the base plate 20 back to themagnet 18 through its bottom pole. A second magnetic circuit extendsfrom the top pole of the magnet 19 to the spring plate end 44 throughthe arm 48 and mirror 12, across a gap to the same stator 22 and throughthe base plate 20 back to the magnet 19 through its bottom pole. A thirdmagnetic circuit extends from the top pole of the magnets 18 to thespring plate end 42, through the arm 46 and mirror 12, across a gap tothe other of the stators 22 and through the base plate 20 back to themagnet 18 through its bottom pole. A fourth magnetic circuit extendsfrom the top pole of the magnet 19 to the spring plate end 44 throughthe arm 48 and mirror 12, across a gap to such other stator 22 andthrough the base plate 20 back to the magnet 19 through its bottom pole.

When a periodic drive signal such as a square wave is applied to theoppositely wound coils 26, magnetic fields are created which cause themirror 12 to oscillate back and forth about the axis of rotation 16.More particularly, when the square wave is high for example, themagnetic fields set up by the magnetic circuits through the stator 22and magnets 18, 19 cause an end 66 of the mirror to be attracted to thestator 22. At the same time, the magnetic field created by the magneticcircuits extending through the other stator 22 and the magnets 18, 19cause the opposite end 68 of the mirror to be repulsed by the stator 22.Thus, the mirror is caused to rotate about the axis of rotation in onedirection. When the square wave goes low, the magnetic field created bythe first stator 22 repulses the end 66 of the mirror whereas the otherstator 22 attracts the end 68 of the mirror so as to cause the mirror 12to rotate about the axis 16 in the opposite direction. A periodic squarewave causes the mirror 12 to rotate in one direction then another in anoscillatory manner.

Mirror Position Detection

FIG. 4 shows the range of motion of the mirror 12 along its oscillatorypath. In the relaxed state the mirror 12 rests flat in a levelorientation 70. At one extreme the mirror 12 is deflected rotationallyabout the axis 16 by an angle +θ to assume a first extreme orientation72. At the other extreme the mirror 12 is deflected rotationally aboutthe axis 16 by an angle -θ to assume a second extreme orientation 74.

FIG. 5 shows the spring plate 14 underside 76. Two piezoelectric sensors78, 80 are mounted to the underside 76 opposite the mirror 12. Eachsensor 78, 80 is equidistant from the mirror's axis of rotation 16.Accelerated motion of a respective sensor 78/80 induces an electricalvoltage across the component piezoelectric material. Changes inacceleration occur as changes in voltage. Zero acceleration correspondsto a constant "zero level" voltage output (e.g., ground or some voltagebias level). Sensor 78 generates an output voltage signal 82. Sensor 80generates a output voltage signal 84.

As the mirror 12 moves along its deflection path from one extremeorientation 72 to another extreme orientation 74, the mirror acceleratesand decelerates. As the mirror 12 approaches the first extremeorientation 72 the mirrors slows then reverses direction. Thiscorresponds to a peak acceleration point. Similarly, as the mirror 12approaches the second extreme orientation 74 the mirror slows again andreverses direction. This also corresponds to a peak acceleration point.The two peaks correspond to accelerations of opposite magnitude. Themirror 12 achieves maximum velocity as it moves into the levelorientation 70. Such maximum velocity corresponds to a zero accelerationpoint as the mirror stops speeding up and begins slowing down. A zeroacceleration point occurs each time the mirror 12 assumes the levelorientation 70.

As the zero acceleration point approaches, the sensor 78 voltagemagnitude reduces to zero level (e.g., ground or bias voltage level).Similarly, the sensor 80 voltage magnitude also reduces to zero level.The direction of voltage change for the two sensors, however, varies.One is going from positive to negative polarity, while the other isgoing from negative to positive polarity. The zero crossover occurs atthe same time for each sensor. By monitoring the zero crossovers one candetect when the mirror 12 is in the level orientation 70.

In a preferred embodiment the sensor output signals 82, 84 are input toa differential amplifier 88. The amplifier 88 performs a common moderejection outputting a difference signal 90, which is the differencebetween the voltages of the two signals 82, 84. The piezoelectricsensors 78, 80 respond to motion acceleration in any direction. Ideallythe acceleration is only rotational about the axis of rotation 16.However, the scanner 10 itself is moving in some applications. Toprevent such common motion from casing false zero crossover detections,the difference between the sensor output signals 82, 84 is monitored.The differential amplifier 88 subtracts out the voltage componentcorresponding to a common motion direction of the two sensors 78, 80.Any motion occurring along the axis of rotation 16 is sensed by eachsensor 78, 80 having opposite directions and is not subtracted out. Anymotion occurring along another axis is sensed in common by the sensors78, 80 and is subtracted out. As a result, only the voltage componentscorresponding to motion in the rotational direction about axis 16 causesa zero crossover to be detected. The difference signal varies over timeaccording to the oscillating path of the mirror 12 about the axis ofrotation. The difference signal 90 exhibits a zero level at each zeroacceleration position of the mirror (i.e., the level orientation 70).Thus, the difference signal 90 indicates the phase position of themirror.

Virtual Retinal Display

FIG. 6 shows a block diagram of a virtual retinal display 100 using amechanical resonant scanner embodiment of this invention. The display100 receives image data from a computer device, video device or otherdigital or analog image data source. Light generated by the display 100is altered according to the image data to scan an image into the retinaof a viewer's eye E.

The retinal display 100 generates and manipulates light to create coloror monochrome images having narrow to panoramic fields of view and lowto high resolutions. Light modulated with video information is scanneddirectly onto the retina of a viewer's eye E to produce the perceptionof an erect virtual image. The retinal display is suitable for hand-heldoperation or for mounting on the viewer's head.

The retinal display 100 includes an image data interface 111, a lightsource 112, an optics subsystem 114, a scanning subsystem 116, an exitpupil expanding apparatus 118, and an eyepiece 120. The image datainterface 111 receives a video or other image data signal, such as anRGB signal, NTSC signal, VGA signal or other formatted color ormonochrome video or image data signal. The image data interface 111extracts color component signals from the received image data signal. Inan embodiment in which an image data signal has embedded red, green andblue components, the red signal is extracted and routed to a firstmodulator for modulating a red light source output. Similarly, the greensignal is extracted and routed to a second modulator for modulating agreen light source output. Also, the blue signal is extracted and routedto a third modulator for modulating a blue light source output. Theimage data signal interface 111 also extracts a horizontalsynchronization component and vertical synchronization component fromthe received image data signal. In one embodiment, such signals definerespective frequencies for a horizontal scanner and vertical scannerdrive signals. Such synchronization components or drive signals arerouted to the scanning subsystem 116.

The light source 112 includes a single or multiple light sources. Forgenerating a monochrome image a single monochrome source typically isused. For color imaging, multiple light sources are used. In oneembodiment red, green and blue light sources are included. Exemplarylight sources are colored lasers, laser diodes or light emitting diodes(LEDs). Although LEDs do not output coherent light, lenses are used inone embodiment to shrink the apparent size of the LED light source andachieve flatter wave fronts. In another LED embodiment a single modemonofilament optical fiber receives the LED output to define a pointsource which outputs light approximating spatially coherent light. Thelight sources or their output beams are modulated according to the inputimage data signal content to produce light which is input to the opticssubsystem 114. In one embodiment the emitted light is coherent. Inanother embodiment the emitted light is noncoherent.

The optics subsystem 114 serves as an objective to focus the light. Forsome embodiments in which noncoherent light is received, the opticssubsystem 114 also collects the light. Left undisturbed the light outputfrom the optics subsystem 114 converges to a focal point then divergesbeyond such point. When the converging light is deflected, however, thefocal point is deflected. The pattern of deflection defines a pattern offocal points. Such pattern is referred to as the intermediate imageplane 115.

Prior to the image plane 115 is the scanning subsystem 116. The scanningsubsystem 116 deflects the light and the ensuing focal point to definethe intermediate image plane 115 of focal points. Typically the light isdeflected along a raster pattern. In one embodiment the scanningsubsystem 116 includes a resonant scanner 10 for performing horizontalbeam deflection and a galvanometer for performing vertical beamdeflection. The scanner 10 serving as the horizontal scanner receives adrive signal having a frequency defined by the horizontalsynchronization signal extracted at the image data interface 111.Similarly, the galvanometer serving as the vertical scanner receives adrive signal having a frequency defined by the vertical synchronizationsignal VSYNC extracted at the image data interface. Preferably, thehorizontal scanner 10 has a resonant frequency corresponding to thehorizontal scanning frequency.

The exit pupil expanding apparatus 18 is optional and when presentcoincides with the intermediate image plane 115. The apparatus 18 servesto spread light over a larger surface area at the eyepiece 20. The exitpupil expanding apparatus 18 generates multiple closely spaced (oroverlapping) exit pupils and/or enlarges the exit pupil(s). Adiffractive optical element embodiment generates multiple exit pupils. Afiber-optic face plate embodiment, lens array embodiment or diffuserembodiment enlarges a single exit pupil. The light output from the exitpupil expanding apparatus 18 travels to the eyepiece 20. The expandedexit pupil(s) occur slightly beyond the eyepiece 20 at a location wherea viewer positions the pupil of their eye E.

The eyepiece 20 typically is a multi-element lens or lens system. In analternative embodiment the eyepiece 120 is a single lens. The eyepiece120 contributes to the location where an exit pupil 21 of the retinaldisplay 100 forms. The eyepiece 120 defines an exit pupil at a knowndistance d from the eyepiece 120. Such location is the expected locationfor a viewer's eye E. The eyepiece 120 preferably is positioned at onefocal distance from the intermediate curved image plane 115. In analternative embodiment the relative distance between the image plane 115and eyepiece is variable. In the case where the relative distance isslightly less than one focal length, the size and apparent depth of theimage formed in the viewer's eye changes.

Correcting for Mirror/Drive Signal Phase Errors

To correct for phase errors between a scanner's drive signal and themirror 12 oscillation, a phase locking circuit 200 is implemented asshown in FIG. 7. The phase locking circuit 200 is formed by a mechanicalresonant scanner 10, a phase locked loop circuit (PLL) 202 and a lowpass filter 201. The PLL 202 includes a phase comparator 203 and avoltage controlled oscillator 205. As described above, the resonantscanner 10 receives incoming light and reflects outgoing light at themirror 12. The scanner 10 also receives a drive signal 204 forenergizing electromagnetic coils 26 which deflect the scanner's mirror12. The scanner 10 generates the difference signal 90 at thedifferential amplifier 88.

The phase comparator 203 receives the difference signal 90 and thescanner's drive signal 204. The difference signal 90 serves as areference signal and corresponds to the position phase of the mirror 12.The phase of the drive signal is adjusted to align (e.g., coincide or be180° out of phase) with the phase of the difference signal 90. The phasecomparator 203 output is received by the low pass filter 201 then inturn to the voltage controlled oscillator (VCO) 205. The adjusted drivesignal is output from the VCO 202 to the scanner 10. In the embodimentshown in FIG. 7 the phase locking circuit is running at the drive signal204 frequency.

FIG. 8 shows an alternate embodiment in which a pixel clock signal isderived. The pixel clock signal is routed to the light source subsystem112 to serve as a horizontal synchronization signal. In this embodimentthe phase locking circuit 200' is operating at a frequency greater thanthe drive signal 204 frequency. Thus, the output from the VCO 205 isdivided by some factor N to obtain the drive signal 204 frequency. Adivider 210 performs the divide by N operation. The pixel lock also isderived from the VCO 205 output by dividing the signal by an appropriatefactor M at a divider 212.

For either of the FIG. 7 or FIG. 8 embodiments when the resonancefrequency of the scanner varies due to environmental impacts, such aschanges in temperature, the drive signal 204 is adjusted to align thedrive signal phase to the altered mirror position phase. For the FIG. 8embodiment the phase of the pixel clock is similarly adjusted.

Meritorious and Advantageous Effects

Although a preferred embodiment of the invention has been illustratedand described, various alternatives, modifications and equivalents maybe used. Therefore, the foregoing description should not be taken aslimiting the scope of the inventions which are defined by the appendedclaims.

What is claimed is:
 1. An optical resonant scanner, comprising:areflective surface for deflecting light, the reflective surfacealternately rotating about an axis of rotation in a first direction anda second direction, wherein the rotation in the first direction occursbetween a first extreme rotational position and a second extremerotational position, and wherein the rotation in the second directionoccurs between the second extreme rotational position and the firstextreme rotational position, wherein the alternate rotation about theaxis of rotation defines an oscillatory motion of the reflectivesurface, and wherein the reflective surface undergoes zero accelerationduring said oscillatory motion at a third rotational position occurringbetween the first extreme rotational position and the second extremerotational position; a first sensor generating a first output signalvarying as a function of accelerated movement of the first sensor andhaving a zero level in the absence of accelerated movement; a secondsensor generating a second output signal varying as a function ofaccelerated movement of the second sensor and having a zero level in theabsence of accelerated movement, wherein the first sensor and secondsensor are equidistant from the axis of rotation and move with thereflective surface, and wherein the first sensor and second sensor areoriented to generate respective first output signal and second outputsignal components at a 180 degree phase difference attributable toacceleration about the axis of rotation; means for differencing thefirst output signal and second output signal to generate a differencesignal having a zero level whenever the reflective surface is at thethird rotational position.
 2. The scanner of claim 1 in which thedifferencing means comprises a differential amplifier performing commonmode rejection on the first output signal and second output signal. 3.The scanner of claim 1, in which the oscillatory motion is characterizedby a first frequency, and further comprising:an actuation circuit formoving the reflective surface in said oscillatory motion, the actuationcircuit receiving a drive signal; and a phase locked loop for lockingphase of the drive signal to phase of the oscillatory motion so as to beeither one of in phase or at a 180 degree phase difference.
 4. Thescanner of claim 3, in which the drive signal has a frequency equal tothe first frequency.
 5. A method for identifying a known position of areflective surface within an optical resonant scanner, the reflectivesurface deflecting light along a scan path, the reflective surfacealternately rotating about an axis of rotation in a first direction anda second direction, wherein the rotation in the first direction occursbetween a first extreme rotational position and a second extremerotational position, and wherein the rotation in the second directionoccurs between the second extreme rotational position and the firstextreme rotational position, wherein the alternate rotation about theaxis of rotation defines an oscillatory motion of the reflectivesurface, and wherein the reflective surface undergoes zero accelerationduring said oscillatory motion at a third rotational position occurringbetween the first extreme rotational position and the second extremerotational position; the method comprising the steps of:generating afirst output signal at a first sensor, the first output signal varyingas a function of accelerated movement of the first sensor and having azero level in the absence of accelerated movement; generating a secondoutput signal at a second sensor, the second output signal varying as afunction of accelerated movement of the second sensor and having a zerolevel in the absence of accelerated movement, wherein the first sensorand second sensor are equidistant from the axis of rotation and movewith the reflective surface, and wherein the first sensor and secondsensor are oriented to generate respective first output signal andsecond output signal components at a 180 degree phase differenceattributable to acceleration about the axis of rotation; anddifferencing the first output signal and second output signal togenerate a difference signal having a zero level whenever the reflectivesurface is at the third orientation.
 6. The method of claim 5 in whichthe step of differencing comprises performing common mode rejection onthe first output signal and second output signal.
 7. The method of claim5, in which the oscillatory motion is characterized by a resonantfrequency, and in which an actuation circuit moves the reflectivesurface in said oscillatory motion, the method further comprising thestep of receiving a drive signal; andlocking phase of the drive signalto phase of the oscillatory motion so as to be either one of in phase orat a 180 degree phase difference.
 8. An optical resonant scanner,comprising:a reflective surface for deflecting light, the reflectivesurface alternately rotating about an axis of rotation in a firstdirection and a second direction, wherein the rotation in the firstdirection occurs between a first extreme rotational position and asecond extreme rotational position, and wherein the rotation in thesecond direction occurs between the second extreme rotational positionand the first extreme rotational position, wherein the alternaterotation about the axis of rotation defines an oscillatory motion of thereflective surface, and wherein the reflective surface undergoes zeroacceleration during said oscillatory motion at a third rotationalposition occurring between the first extreme rotational position and thesecond extreme rotational position; a first sensor generating a firstoutput signal varying as a function of accelerated movement of the firstsensor and having a zero level in the absence of accelerated movement; asecond sensor generating a second output signal varying as a function ofaccelerated movement of the second sensor and having a zero level in theabsence of accelerated movement, wherein the first sensor and secondsensor are equidistant from the axis of rotation and move with thereflective surface, and wherein the first sensor and second sensor areoriented to generate respective first output signal and second outputsignal components at a 180 degree phase difference attributable toacceleration about the axis of rotation; and a difference signal derivedfrom the first output signal and second output signal and having a zerolevel whenever the reflective surface is at the third rotationalposition.
 9. The scanner of claim 8, in which the oscillatory motion ischaracterized by a resonant frequency, and further comprising: a drivesignal which energizes an electromagnet for actuating the oscillatorymotion; anda phase locked loop for locking phase of the drive signal tophase of the oscillatory motion so as to be either one of in phase or ata 180 degree phase difference.